Fuel bundles for boiling water reactors are known. They include a plurality of upstanding parallel fuel rods supported on a lower tie plate with at least some of the fuel rods extending to and fastened to an upper tie plate. A fuel bundle channel surrounds the fuel rods at the lower tie plate and extends to the region of the upper tie plate. In operation, water flows into the fuel bundle through the lower tie plate and flows upward of the fuel bundle in a confined flow path within the channel generating steam. Water and steam exit the fuel bundle from the upper tie plate.
The fuel rods are long, slender and flexible. In the dynamics of steam generation, these rods would undergo vibration and come into abrading contact one with another. Consequently, so-called fuel bundle spacers are utilized. These spacers surround each fuel rod at spaced apart vertical locations along the length of the fuel bundles and essentially maintain the fuel rods in their designed side-by-side relation interior of the fuel bundle channel.
As pertinent to this disclosure, the use of so-called ferrule spacers in the upper two phase region of the fuel bundle is utilized. The ferrules in ferrule spacers constitute discrete cylinders having typically paired stops on one side of the ferrules and fuel rod biasing springs on the other side of the ferrules. The inside diameter of the ferrules exceeds the outside diameter of the fuel rods so that the fuel rod when biased by the spring is centered with respect to the ferrule. When the ferrules are held together in a matrix, a rigid and highly satisfactory spacing of the fuel rods results.
It has been discovered that ferrule spacers in combination with fuel rods in the upper two phase region of a fuel bundle have a beneficial effect on critical power performance. Such spacers tend to augment the required liquid film on the surface of the steam generating fuel rods. The maintenance of a liquid film coating prevents a condition known as "dryout" on the surfaces of the fuel rods. The onset of dryout occurs at the first location where the liquid film on any fuel rod is completely evaporated, i.e., the film thickness equals zero. This phenomenon will occur just upstream of a spacer before the liquid film thickness is augmented by passing through the spacer. (Therefore, the performance of spacers 2 and 3 thicken the liquid films that might experience dryout just upstream of spacers 1 and 2, respectively. Such a condition can lead to overheating of the cladding with resultant damage to the fuel rods. Unfortunately, all spacers to some extent create pressure drop. Pressure drop in the upper two phase region of the fuel bundle is preferably to be avoided as such pressure drop contributes to both thermal hydraulic and nuclear thermal hydraulic instabilities.
Swirl vanes may be used in combination with boiling water reactors in the upper two phase region of fuel bundles in such reactors. As relevant to this disclosure, at least the following three disclosures should be considered.
Swirl vanes were first proposed in combination with a boiling water reactor fuel rods not confined to fuel bundles. These swirl vanes were the same length as the fuel rods and placed interstitially of the fuel rods. As both the modern expedient of fuel bundles was omitted and pressure drop was high, these designs had no commercial significance.
A spacer made entirely of swirl vanes is disclosed in Johansson U.S. patent application Ser. No. 07/702,212 filed May 17, 1991 (now issued U.S. Pat. No. 5,186,891 issued Feb. 16, 1993), entitled SWIRL VANES AND INCONEL SPACER. In this design, swirl vanes are co-extensive with the height of the spacer and occupy each and every spatial interval between fuel rods. As a consequence, high pressure drop is experience in such a spacer.